This invention relates to a method and microfabricated device for sorting cells or particles by size, charge or other identifying characteristics, for example, characteristics that can be optically detected. The invention includes a fluorescence activated cell sorter (FACS), and methods for analyzing and sorting cells by measuring a signal produced by an optically-detectable (e.g., fluorescent, ultraviolet or color change) reporter associated with the cells. The methods and apparatus of the invention allow for high sensitivity, no cross-contamination, and lower cost than conventional FACS machines. In preferred embodiments, cell sorting is performed on a microfabricated chip with a detection volume of approximately 1 to 1,000,000 femtoliters (fl), preferably about 200 to 500 fl, and most preferably about 375 fl. Sorting occurs immediately after detection. In a particular embodiment, the inlet and collection wells are incorporated on the same chip.
Sorters of the invention can function as stand-alone devices or as components of integrated microanalytical chips, and can be disposable. Living cells with a distinguishing characteristic, such as E. coli cells expressing a fluorescent protein, can be efficiently separated from cells lacking this characteristic. Furthermore, the cells remain viable after being extracted from the sorting device. An advantage of the invention is that it can be applied to various aspects of chemical and biological studies, e.g., cell sorting, enzyme catalysis and molecular evolution (1).
The references cited herein are referred to numerically, and are appended in a Bibliography below. All of the references are incorporated herein in their entirety.
Harrison et al. (39) disclose a microfluidic device which manipulates and stops the flow of fluid through a microfabricated chip, so that a cell can be observed after it interacts with a chemical agent. The cells and the chemical agent are loaded into the device via two different inlet channels which intersect with a main flow path. The flow of the fluid is controlled by a pressure pump or by electric fields (electrophoretic or electro-osmotic) and can be stopped so that the cells can be observed, after they mix and interact with the chemical. The cells then pass through the main flow path, which terminates in a single common waste chamber. Harrison et al. do not provide a device or method for sorting cells, nor do they suggest or motivate one having ordinary skill in the art to make and use any such device. On the contrary, cells are mixed with chemicals, observed, and are discarded as waste.
Conventional flow cell sorters, such as FACS, are designed to have a flow chamber with a nozzle and use the principle of hydrodynamic focusing with sheath flow to separate or sort biological material such as cells (2–7). In addition, most sorting instruments combine the technology of ink-jet writing and the effect of gravity to achieve a high sorting rate of droplet generation and electrical charging (8–10). Despite these advances, many failures of these instruments are due to problems in the flow chamber. For example, orifice clogging, particle adsorption and contamination in the tubing may cause turbulent flow in the jet stream. These problems contribute to the great variation in illumination and detection in conventional FACS devices. Another major problem is known as sample carryover, which occurs when remnants of previous specimens left in the channel back-flush into the new sample stream during consecutive runs. A potentially more serious problem occurs when dyes remain on the tubing and the chamber, which may give false signals to the fluorescence detection or light scattering apparatus. Although such systems can be sterilized between runs, it is costly, time consuming, inefficient, and results in hours of machine down time for bleaching and sterilization procedures.
Similarly, each cell, as it passes through the orifice, may generate a different perturbation in response to droplet formation. Larger cells can possibly change the droplet size, non-spherical cells tend to align with the long axis parallel to the flow axis, and deformable cells may elongate in the direction of the flow (9, 10). This can result in some variation in the time from the analysis to the actual sorting event. Furthermore, a number of technical problems make it difficult to generate identically charged droplets, which increases deflection error. A charged droplet may cause the next droplet of the opposite polarity to have a reduced charge. On the other hand, if consecutive droplets are charged identically, then the first droplet might have a lower potential than the second droplets, and so on. Yet, charged droplets will have a defined trajectory only if they are charged identically. In addition, increasing droplet charges may cause mutual electrostatic repulsion between adjacent droplets, which also increases deflection error. Other factors, such as the very high cost for even modest conventional FACS equipment (on the order of $250,000), the high cost of maintenance, and the requirement for trained personnel to operate and maintain the equipment have been among the main considerations that hinder this technology and its widespread accessibility and use (10). Even though the field of flow cytometry has been extensively exploited in the development of cell sorting devices, significant problems persist and remain to be addressed. Thus, there is a need for improved methods and machines for cell sorting which are fast, efficient, cost-effective and disposable.